There are many known bag machines. One style is a rotary drum machine. Rotary drum machines are well known, and found in U.S. Pat. Nos. 6,117,058, 4,934,993, 5,518,559, 5,587,032 and 4,642,084 (each of which is hereby incorporated by reference). Bag machine, as used herein, includes a machine used to make bags such as draw tape bags, non-draw tape bags, and other bags.
A detailed description of the operation of rotary bag machines may be found in the patents above, or in prior art commercially available machines such as the CMD 1270GDS or 1552ED, but their general operation may be seen with respect to FIG. 1. A prior art rotary bag machine 100 continuously processes a web 201 using an infeed section 203, a sealing section 210, and a perforation section 118. An output section may be included. The web is provided through infeed section 203, to sealing section 210, and then to perforation section 118. A controller 121 controls the servo motors (or other motors) that control where seals and perforations are made.
Infeed section, as used herein, includes a section that receives film from a roll, and may be simply a nip, or other components such as dancers, etc. Perforation section, as used herein, includes a section that applies a perforation to a film, and may include rotary knives. Sealing section, as used herein, includes a section that applies one or more seals to a film to form bags, and may include a rotary drum. Output section, as used herein, includes processing stations that act on a web downstream of the seals being formed, such as winders, folders, etc. Processing station, as used herein, includes any device that operates on the film, such as sealing, folding, perforating, winding, etc.
Sealing section 210, including drum 208, impart a plurality of successive seals to form bags (one or more seals may be used to form each bag). Plurality of successive seals, as used herein, includes more than one seal or group of seals that form a bag.
The prior art of FIG. 1 provides that after web 201 leaves drum 208 (in sealing section 210) it is directed to a rotary knife (part of perforation section 118), which creates a perforation between bags, or could separate adjoining bags. When the bags are end to end bags, the perforation is placed close to the single seal such that when the bags are separated, the perforation and the perforated end are the top of one bag, and the seal is the bottom of the adjoining bag. Ideally, the perforation is close to the seal to reduce waste, although this is difficult in practice. The distance between the seal and the perforation is called the skirt length.
When bags are formed side to side, the perforation is made between the pair of seals. Thus, there are skirt lengths on either side of the perforation. A seal is needed on both sides of the perforation, since the side of both bags should be sealed. The web between the pair of seals is wasted. Thus, the pair of seals should be close to one another to reduce waste, although this requires the perforation to be precisely located, which is difficult in practice. For example, variation due to tension, film gauge variation, machine variations etc., occasionally causes seals to get cut off, or the distance between a seal and perforation to be too great.
Controller 121 is connected to the various components to control speed, position, etc. Controller, as used herein, includes digital or analog components, and related software, inputs and outputs, located together or separately, that cooperates to control one or more portions of a machine.
Sensors have been used to detect seals prior to the formation of the perforation to help form the perforation in the correct location. Sensor, as used herein, includes sensors that sense seals, burn marks, holes, ink marks, etc. Sensing the seal has proven to be difficult. One prior art example of a system that sensed seals is described in U.S. Pat. No. 6,792,807, hereby incorporated by reference. Another prior art patent, U.S. Pat. No. 5,447,486 uses printed marks or marks created by the seal bar to sense the location of the seal to try and register the perforation to the registration. If the perforation is placed too close to one side seal, then the seal may be cut off, rendering the bag useless. Other systems that attempt to control the skirt length are described in U.S. Pat. Nos. 5,660,674 and 5,701,180. However, the prior art methods often provide false positives or miss the seals. The prior art located the sensors at various locations, including near the drum, or near the knife, but provided a single sensor.
Accordingly, a method and machine for making bags that provides for accurate detection of seals are desired.